A stepping motor may fall into different categories such as permament-magnet (PM) and variable-reluctance (VR) types. When incremental output motion is desired, stepping motors provide a logical link between digital information and mechanical translation. For some time, these stepping motors (both the permanent magnet and variable reluctance types) have been used as output devices for various kinds of incremental-motion control systems. In fact, the importance of stepping motors has become even more prominent since control commands are currently being provided more frequently in digital form.
For many situations, the system load consists mainly of an inertia load, with or without a moderate amount of friction. The system requirement most often sought after is that, given a digital command, the load must be moved to a new position within minimum time and without overshoot since overshoot invariably leads to oscillation or hunting. This problem is frequently encountered in the design of modern computer peripherals. For example, in the case of a high-speed single-element serial incremental printer, the printing head has to be positioned at very high speed and with great accuracy.
To understand a fundamental problem inherent in the operation of a stepping motor, it is necessary to understand some features of the construction and operation of a stepping motor. More particularly, a stepping motor includes a rotor which cooperates with stator windings. Provision is made for energizing one or more sets of the stator windings so that the stepping motor can be "detented" in the sense that its rotor moves to a position at which the vector sum of the torques created by the stator fields is zero. In the case of permanent magnet types, this means that the rotor field is aligned with the stator field. For variable reluctance types, this means that the magnetic path of the stator-generated flux is of minimum reluctance.
If the first actuated set of windings is de-energized and the adjacent set is next energized, the rotor will tend to move towards the neighboring detent position. If continuous motion is desired, the windings are switched on sequentially so that there is a "revolving" or mobile field which moves in discrete steps.
At low switching speeds, the rotor follows the field in a go-stop manner with some oscillation or hunting every time it comes to a stop. At high switching speeds, the movement of the rotor will change from discrete motion to a continuous forward motion often referred to as "slew". Since subsequent switching pulses arrive when the motor is running at a different velocity, the performance of the motor becomes unpredictable. This gives a qualitative description of an industry-wide problem which tends to limit the use of stepping motors.
Although the stepping motor is inherently suited for driving loads in an incremental fashion, it is plagued, in practical applications, by the following difficulties:
(1) high speed and satisfactory resolution cannot be attained simultaneously; PA1 (2) the presence of oscillation or hunting after the rotor has reached a destination; and PA1 (3) the possibility of loss of synchronization during high speed running.
The first difficulty listed above has not been solved by any other method. For example, a 15.degree./step 720 step/sec. motor can run at 1800 rpm but cannot have a resolution of 1.8.degree./step. On the other hand, a 1.8.degree./step motor must run at 6000 step/sec. to achieve 1800 rpm. Gear reduction, aside from its cost, can improve resolution, but must sacrifice maximum speed.
The second difficulty listed above has been alleviated to some extent by various kinds of damping techniques. Among these techniques is mechanical damping which suffers from an accompanying loss of motor performance in addition to high cost. Electronic or switching damping techniques which are also known sometimes give satisfactory results, but are extremely difficult to implement. For example, a so-called delayed-last-step technique is applicable only if overshoot consistently approaches 100% of the stepping angle. On the other hand, a so-called back phasing (bang-bang) damping technique works only if the terminal velocity is known each time the motor stops. In general, it can therefore be stated that, with the present state of art, most incremental control systems have to be compromised in performance to some extent due to inadequate damping.
The third difficulty listed above has been resolved by adding an "encoder" to the associated control circuit. The encoder senses the position and/or the speed of the rotor and then sends this information to the motor control circuit. Sensing the difference between what is desired and what is actually attained, the control circuit takes corrective action. However, there is substantial increase in cost if an encoder (also known as closed-loop control) is employed.
In the field of stepping motor controls, there will be found a wide variety of patents, as will be shown hereinafter. However, these patents fail to reveal the principal features of the present invention. Some of the patents which have been located will next be discussed for purposes of establishing a further environment for the disclosure which follows hereinafter.
S. Inaba in U.S. Pat. No. 3,579,279 deals with the problem of "loss of synchronization" when a stepping command changes abruptly. The circuit smooths out the rapid changes so that a sudden start or stop is prevented from occurring. The object of the control provided in this patent is to avoid extremely large overshoots which might bring the rotor out of synchronization with a command. This technique blindly slows down the clock during deceleration. The motion of the rotor can only be expected to follow the command faithfully without erroneous movements. As for sub-step oscillation elimination, the method is completely ineffective. This control is aimed at controlling the motor when large multi-revolution distances are to be travelled. For incremental motions which are only a few steps or a fraction of a step, this technique is totally inapplicable.
Along the same lines, C. J. Clark, Jr. in U.S. Pat. No. 3,818,261 discloses a technique and apparatus which is essentially the same as the Inaba apparatus.
K. E. Hendrickson in U.S. Pat. No. 3,732,480 discloses driving a stepping motor with variable width pulses. The width of each pulse is predetermined by solving a set of simultaneous motion equations with the use of digital computers. The physical implementation consists of a time base generator whose output voltage is constantly compared with a set of preset constant voltages. Whenever a coincidence occurs, a new pulse is sent to the motor winding. This technique attempts to control the motion by frequency means. This, in itself, distinguishes from the present invention. According to the example described in the Hendrickson patent, the reference voltage resolution requires three digits. This means that the technique is difficult to apply without costly labor. Furthermore, the disclosure states that the assumption of constant inductance is not valid when winding current approaches its rated value. This means that the motor cannot be driven at its full capacity and therefore performance deteriorates.
R. A. McSparran in U.S. Pat. No. 3,787,727 takes into account the following control techniques in half-step operation: (a) acceleration period can be reduced by temporarily increasing the winding currents and (b) overshoot can be reduced by slowing down on the driving clock. Since torque characteristics are quite difficult when a stepping motor is driven to full step or half step, two steps of start and stop control are used to compensate for the difference. The clock rate is governed by the position feedback signal. Although, this technique uses two levels of voltage for motor windings, it involves essentially a digital drive and has nothing to do with the instant invention.
B. Sawyer in U.S. Pat. No. 4,009,428 reveals and attempts to achieve a trapezoidal velocity profile. This, in itself, distinguishes from the instant invention. Further, this patent assumes the motor to be an idealized A. C. synchronous motor. This further distinguishes from the instant invention. Sawyer, to achieve a burst of constant acceleration, provides that the revolving field must lead the motor by a constant angle. This means that the frequency of the driving clock must vary continuously with time during acceleration and deceleration. To this end, the Sawyer circuit incorporates a ramp generator and a variable frequency pulse generator. The Sawyer circuit is thus completely different from the circuit of the instant invention which uses a constant and switchable clock to drive a digital-to-analog converter. Furthermore, the lack of physical characterization of the motor inevitably leads to accuracy problems in the Sawyer solution. In other words, since the motor torque depends on the physical construction and the saturation properties of the magnetic material, a simple set of sine-cosine currents proposed in accordance with this patent will not produce the desired constant acceleration and deceleration in actual applications.
L. Thompson in U.S. Pat. No. 3,328,658 discloses a method typically known as bang-bang control as discussed hereinabove. The difficulty with this method is that the timing of the braking pulse must depend upon the terminal velocity of the rotor as well as the lead angle immediately before braking is applied.